Closing mechanisms with parts moved by an electric motor are put to various uses--for example, in motor vehicles, for the electrical operation of side windows and sliding roofs. With control movements for these closing mechanisms and in particular with those closing movements which are automatic (i.e. when automatic functions are implemented), there is the risk that parts of the body or objects may get squeezed. Therefore, regulations prescribe that some of these closing mechanisms must be fitted with some kind of squeeze protection, although this should be a general provision for this type of closing mechanism for reasons of safety. To guarantee that squeeze protection will work, for example, the closing movement of the electromotive part can be monitored for any obstruction either by optical detection or by determination and evaluation (of the time history) of the squeezing force exerted when squeezing. Known sensors required for direct determination of the squeezing force make this an extremely expensive option (for example, force sensors or torque sensors). With know sensor, the squeezing force or its time history, is determined indirectly, by recording at least one performance characteristic of the motor typical for the load on the electric motor, to act as a measured variable (for example, in DE 30 34 118 C2, by recording the speed of the motor, or in DE 44 42 171 A1 by recording the current of the motor). The instantaneous measured value of the performance characteristic for the motor is evaluated by comparing it with a reference measurement or several reference measurements (i.e. the relative changes of the instantaneous measured variable are evaluated with regard to a past reference value of time or place for the same measured variable). Should a given limit be exceeded (tripping threshold), a squeezing incident will be assumed to have taken place, which will initiate a specific reaction from the closing mechanism (for example, the motor drive will go into reverse or the current to the motor will be switched off). However, it is a problem that there is often insufficient monitoring of the closing movement, and the associated lack of adequate squeeze protection, immediately after the closing movement has been activated (i.e. when the electric motor starts up). In this starting phase, there is no reference measurement and no suitable reference value available and thus, no squeeze protection. The closing movements which are a particular problem are those which immediately follow opening movements, as there is first an equalization of drive play ("compensation for play") in the closing mechanism (especially for the play in the motor/gearing). The result of this is that often, the distinction cannot be made between an increase in load which follows successful compensation for play (i.e. a build up of force after compensation for play, to overcome the static friction limit) and an actual squeezing incident.
One way to avoid this problem is to add a short opening movement before each closing movement (as described, for example in DE 35 14 223 C2 or DE 33 46 366 C2). This ensures firstly that the compensation for play and the subsequent increase in load (tensioning the mechanical arrangement of the closing mechanisms) is already completed once the electromotive part reaches its original position and that secondly, there is already a suitable reference value available for the evaluation. The disadvantage, however, is that the switching contact provided to activate the control movement (usually in the form of a relay) is put under load twice for each closing movement, which puts an increased strain on the switching contact, which is particularly undesirable in the car industry, on the grounds of the cost involved.
A further option is only to enable the automatic closing movement (automatic function) of the electromotive part, once a defined closing distance has been traveled. The disadvantage here, however, is that it is not possible to activate the automatic function immediately when the closing movement is activated, even if this is what is desired. This invention is based on the task to provide a process which can guarantee effective protection against squeezing, whatever the environmental conditions or operating conditions of the closing mechanism.